In the industrial sector relating to the production of electric batteries there exists a particularly pressing need to obtain, by means of low-cost processes, continuous high-quality lead strips which are then used for manufacturing, by means of conventional stamping or expansion or incision forming processes, the positive and negative grids for batteries.
These grids must have a crystal microstructure which is homogeneous as far as possible, advantageously of the globular or fine-grain type, suitable for ensuring optimum mechanical properties and a high corrosion resistance.
As is known, the abovementioned properties are obtained in an optimum manner by means of rolling of a lead strip with a large thickness, typically 8-18 mm, down to a thickness of about 1 mm.
During the rolling stage the lead alloy in the strip undergoes processes which aim to convert the polycrystalline and relatively non-homogeneous starting structure into crystal structures which have the abovementioned optimum mechanical and corrosion-resistant properties, these processes including, for example, those of homogenization, compaction, fine-grain recrystallization, density increase, grain size reduction and increase of the number of grains.
The present invention therefore relates specifically to the industrial sector for the production of strips with a large thickness, this term being understood as meaning a thickness in the range of 12-18 mm, intended to be subsequently rolled in order to obtain grids with optimum mechanical and corrosion-resistance properties.
It is known, for example, from the U.S. Pat. No. 4,498,519 to use a so-called “twin roll casting” machine for the production of a continuous lead strip, which has a crucible containing a molten lead bath, and two counter-rotating rolls which define between them a slit for communication with the bottom of the crucible, through which the flow of molten lead is intended to pass in order to form the continuous lead strip. In greater detail, the molten lead solidifies when it makes contact with the outer walls of the two rolls, which are suitably kept at a controlled temperature, forming at the outlet of the rollers a continuous strip of the desired thickness.
The main drawback of this machine consists in its high constructional complexity. It in fact requires a complex system of seals, formed with heated shoulders able to contain the molten lead crucible. Moreover, the temperature of the rolls must be regulated in an extremely precise manner in order to allow the strip to leave the rolls completely solidified without, however, an excessive cooling causing occlusion of the through-slit between the rolls or an excessive power consumption for rotation thereof.
Also known is a machine for the continuous production of a lead strip for manufacturing grids for electric batteries, which comprises a rotating drum with an annular cavity for forming the strip. The cavity is closed along an arc of the drum by a flexible steel strip which rotates on the transmission pulleys, forming a seal on top of the side shoulders of the drum. A supply vat for introducing a continuous flow of lead into the annular cavity of the drum is provided between the roll and the strip. The molten lead solidifies along the arc section which it passes over inside the cavity between the drum and the steel strip, emerging tangentially in the form of a continuous strip.
The main drawback of this machine of the known type consists in the limited width of the lead strips which it is able to produce. In fact, the metal strip closing the annular cavity of the drum is subject to deformation in the event of an excessive increase in the width of the cavity and therefore the strip which is to be produced, for example, typically beyond 120 mm. Moreover, an excessive rigidity of the metal strip results in considerable difficulties for rotation thereof on the transmission rolls.
Also known, for example from the U.S. Pat. No. 5,462,109 and U.S. Pat. No. 5,948,566, is a continuous casting machine for forming a lead alloy strip, which comprises a rotating drum with an annular cavity for forming the strip. The cavity is closed along an arc of the drum by a shoe which is arranged on one side of the drum and provided internally with a chamber containing lead in the molten state communicating with the annular cavity. During operation, the rotating drum removes the molten lead from the chamber and cools it along a circumferential arc of about 90° and then releases it tangentially substantially from the top of the drum.
Preferably, cooling rolls are provided for ensuring that the edges of the lead alloy strip are completely solidified before extraction of the strip. The cooling rolls compress for this purpose the strip along the edges against the peripheral wall of the drum. The wall is suitably treated in order to facilitate the formation of lead nucleation points during solidification thereof.
The machines described in the aforementioned U.S. Pat. No. 5,462,109 and U.S. Pat. No. 5,948,566 are intended for the production of strips of very limited thickness, typically of the order of a millimeter, and do not allow the production of strips which have a thickness greater than two millimetres nor those with a large thickness. They are in fact able to transport integrally with the roll only that small layer of lead which solidifies upon contact with the surface of the roll when the latter enters into the molten lead crucible.
Consequently, these machines are entirely unsuitable for the production of strips which are to be rolled with a high rolling ratio for the formation of lead grids with a high grain refinement of the alloy or with optimum mechanical and corrosion-resistance properties.
Finally, a drawback which is common to all the continuous casting machines of the known type described above consists in the fact that the lead does not solidify with a homogeneous crystal and fine-grain structure suitable for ensuring optimum mechanical and corrosion resistance properties. Usually, for the lead, a crystal structure of the globular type, substantially devoid of dendritic formations and with uniformly distributed grains and average dimensions of less than 50 μm, is considered to be optimal.
For example, in order to increase the number of nuclei present in the molten lead bath, the machines described in the U.S. Pat. No. 5,462,109 and U.S. Pat. No. 5,948,566 envisage pitting the outer cooling surfaces of the drums with glass dust, without however this treatment producing satisfactory results.
In view of the speed of production of the continuous strip, the chemical refining techniques, which envisage adding to the lead alloy particles of other already solid components, called inoculating or refining agents (such as copper, selenium and tin), which act as active nuclei in the liquid metal, have proved to be equally unsatisfactory.
Also known in the same industrial sector for the production of electric batteries are machines which are able to form directly a continuous strip of grids from a continuous casting of molten lead.
A known machine of this type is described for example in the U.S. Pat. No. 4,544,014 and envisages continuously casting a molten lead alloy on a rotating drum having formed on the outer peripheral surface a plurality of grooves distributed in accordance with the design of the battery grid which is to be produced. In greater detail, the molten lead alloy is distributed in the grooves of the drum by means of an opening formed in a shoe mounted fixed on the machine and sealingly tightened against the surface of the drum parallel to the axis of rotation of the latter.
The drum is kept at a controlled temperature so as to allow solidification of the lead filaments inside the grooves.
The molten lead remains trapped inside the grooves closed at the top by the shoe and is therefore easily transported by the drum during its rotation.
This machine cannot be modified replacing the grooves with an annular seat in order to form a continuous lead strip, since the problem of transporting the lead arranged inside the seat integrally with the drum would still remain entirely unsolved.